how did scientists discover super volcanoes?

Answer 1

The recognition of disastrous explosive eruptions was the result of many kinds of studies. Geologic mapping, including careful mapping out of patterns of volcanic deposits and faults, isotopic dating and tephrochronology of ash deposits far from original sources, remote sensing, etc.
With respect to the Yellowstone supervolcano, Hayden (1872, 1873) recognized that Yellowstone region was the center of a giant volcanic system. USGS geologist R. Christiansen is largely credited with the most comprehensive studies of Yellowstone's volcanic history (Christiansen and Blank (1972), Christiansen (1984,1998) . These studies outlined a history of three massive caldera-forming eruptions. They clearly used careful geologic mapping as a key research tool.

With respect to the La Garita caldera in the San Juan Mountains of Colorado, USGS geologists Steven and Lipman first mapped out a caldera (1976, USGS Prof. Paper 958) about 40x30 km diameters. subsequent studies have recognized an even larger area of involvement.

With respect to the Toba supervolcano in Indonesia, In 1949 the Dutch geologist Rein van Bemmelen reported that Lake Toba was surrounded by a layer of ignimbrite rocks, and that it was a large volcanic caldera. Later researchers found tuff from the caldera-forming explosion in Malaysia, India and the oceans. Michigan Tech scientists Rose and Chesner calcualted that about 2800 cubic km (670 cubic miles) of material was erupted from Toba about 71,500 years ago.

The Volcanic Explosivity Index (VEI), was devised by Chris Newhall of the U.S. Geological Survey and Steve Self at the University of Hawaii in 1982 to provide a relative measure of the explosiveness of volcanic eruptions.

Answer 2

mostly as a result of the space program. Some of the calderas, such as yellowstone, are only discernable from space.